This study is concerned with the characterization of the plasma cell in various neoplastic states as well as with the elimination or inactivation of blocking and enhancing antibodies. In this connection it is proposed to study the specific in vivo and in vitro effects of a rabbit anti-mouse plasma cell sera (APS) on several murine tumors. The in vitro studies will examine the mitogenic response of murine lymphoid cells from either control or tumor-bearing animals after treatment with APS. The in vivo studies will be concerned with the effect of APS on mice with rapidly growing enhanced tumors. Two basic treatment regimens will be utilized; the tumors will be surgically exicsed and the animals will be treated with APS; tumor-bearing mice will be treated with APS after the tumors have reached a certain size. Correlative fluorescent membrane-labeling of plasma cells with APS will be performed in order to quantitate the number of plasma cells in both control and tumor-bearing animals. The second phase of this study concerns the preparation, purification, and in vitro testing of a rabbit antihuman plasma cell sera (HuAPS). The HuAPS will be studied by standard immunological techniques, including the differential cytotoxic activity against various lymphoid cell populations. Human lymphocytes will be studied as to their ability to respond to various mitogens after treatment with HuAPS. In addition, the quantitation of plasma cells by membrane-labeling will be performed in normal, immunodepressed and tumor-bearing individuals. Since it has been postulated that blocking antibody may inhibit the normal in vivo cellular response of lymphoid cells against tumor cells, a specific humoral immunodepressant may be able to negate or reduce the effect of blocking antibody. Also, through the use of a specific humoral immunosuppressive agent, it should be possible to test whether humoral antibodies to experimental tumors really exert enhancing properties in vivo.